What are the causes of vole population cycles? What are the roles of dispersal, social behavior, and natural selection in controlling population processes in general? The research outlined in this proposal will seek to answer these questions by combining new techniques of study to test three major behavioral hypotheses of population regulation, specifically in voles, but applicable in animal populations in general. Since 1978 my students and I have developed a fence design that overcomes the "fence effect" by letting dispersers leave. We can use this design to trap a group of very highly motivated dispersers. In addition, this design will be used for future manipulations of vole populations. We have also developed a radionuclide-electrophoresis technique to determine relatedness of individual voles in the field. From this technique we can assess the social environment of individuals as well as the heritability of any measurable trait. Using these methods in meadow vole populations (Microtus pennsylvanicus) in eastern Massachusetts we can test three major behavioral hypotheses of population regulation: The polymorphic behavior hypothesis which predicts a high heritability of aggressive behavior in voles; The sociobiological hypothesis which predicts a change in the relatedness of neighboring individuals as density changes; and the outbreeding hypothesis which predicts that changes in density in vole populations will be accompanied by changes in heterozygosity which directly cause changes in aggression that lead to the vole cycle. The experimental design presented will provide the most comprehensive study of vole dispersal, heritability, and relatedness possible and will unambiguously and definitively test the above hypotheses. Understanding the causes of vole cycles will be important in controlling these and similar organisms when they are agricultural or public health pests.